X-ray imaging systems are widely used in medical diagnosis and industrial and security inspection environments. Various prior art X-ray imaging systems are known, including flat-panel radiation imaging sensors. For example, U.S. Pat. No. 4,382,187 to Fraleux et al and U.S. Pat. No. 4,689,487 to Nashiki et al disclose early designs of large area flat-panel radiation imaging sensors which are responsive to incident X-rays and generate output signals representative of a radiation image.
U.S. Pat. No. 5,079,426 to Antonuk et al discloses an indirect conversion X-ray image sensor incorporating amorphous silicon thin film transistor (TFT) switches and a photodiode array. X-rays are detected by a phosphor screen that is placed on the top of the TFT switches and photodiode array. When X-rays interact with the phosphor film, light photons are generated and converted into electronic charges by the photodiode array. The charges are read out via the TFT switches to generate an image. However, problems exist with this prior art sensor. Because the sensor employs a phosphor screen to detect X-rays, blurring occurs due to the fact that the light photons are emitted in all directions and are scattered inside the phosphor screen. This results in a poor image resolution.
In an article entitled "New Solid State Image Pickup Devices Using Photosensitive Chalcogenide Glass Film" by T. Tsukada et al, published in the Proceedings of IEEE International Electron Devices Meeting, 1979, pages 134-136, a solid state imaging sensor is disclosed including a photoconductive selenium film deposited on a N-channel MOSFET switch array made from crystalline silicon. Although this image sensor is suitable for some imaging applications, it is not suited for large area radiation imaging applications due to the difficulties in fabricating a large sensor array on a crystalline silicon wafer.
In an article entitled "Digital Radiology Using Self-Scanned Read Out of Amorphous Selenium" authored by W. Zhao et al, presented at COMP91, Canadian Organization of Medical Physicists, Winnipeg, Manitoba, Canada, Jun. 19, 1991, a flat panel X-ray image sensor is disclosed. The image sensor includes a thick amorphous selenium film (a-Se) on a two-dimensional TFT switch array. The TFT switches are arranged in rows and columns to form a two-dimensional imaging system. Gate lines interconnect the TFT switches in each row while source lines interconnect the TFT switches in each column. The thick selenium film is deposited directly on top of the TFT switch array and a top electrode is deposited on the selenium film. When X-rays are incident on the selenium film and the top electrode is biased with a high voltage, electron-hole pairs are generated and separated by the electric field across the thickness of the selenium film. The holes are driven by the electric field to the pixel electrodes below, which are connected to the respective TFT switches, and stored in the storage capacitor in each pixel. This process results in a charge image being held by the pixels representing the input x-ray image. The signal charges held by the pixel storage capacitors are read out by supplying a positive pulse to the gate lines. When a gate line receives a pulse, the TFT switches in the row turn on, allowing the signal charges on the pixel electrodes to flow to the source lines. Charge amplifiers connected to the source lines sense the charge and provide output voltage signals proportional to the charge and, hence, proportional to the radiation exposure on the selenium film.
One potential risk associated with the prior art system described above, is that TFT switches and peripheral circuits (e.g. charge amplifier multiplexer gate drivers), are susceptible to improper operation and even permanent damage as a result of the high bias voltage on the x-ray conversion layer. Depending on what type of radiation conversion layer is to be used, the electic field strength necessary to adequately bias the radiation conversion layer can range from five to ten volts per micrometer of thickness. For a typical 300 .mu. m thick radiation conversion layer, a three kilovolt bias voltage must be applied. By way of contrast, the gate and bias voltages applied to the TFT switch array and control voltages for peripheral read-out circuitry are typically in the range of from five to ten volts, which is clearly not compatible with the high voltages required to bias the radiation conversion layer.
It is therefore an object of an aspect of the present invention to provide a direct conversion flat panel imaging sensor, which comprises a low voltage driven semiconductor layer for direct radiation conversion and a thin film switch array, and which is capable of providing high sensitivity and high resolution image capture ability.